1. Field of the Invention
The present invention relates to a method for mapping the spatial distribution of the lithologic composition of sediments deposited during a given geologic time interval in a sedimentary basin.
2. Description of the Prior Art
Recent advances in geology, which gave birth during the past twenty years to seismic stratigraphy, then to genetic stratigraphy, have deeply modified understanding of the history of sedimentary filling of sedimentary basins over long time periods and space scales by showing the major influence of two main parameters: the temporal evolution of the topography of the land surface of the basin in relation to the sea level (this topography is referred to hereafter as “bathymetric profile”; the bathymetric profile takes a zero value at any point located at sea level, that is on the coastline) and the sediment supply at the basin boundaries.
Many models and notably deterministic numerical models have been developed to allow obtaining the geometric and lithologic implications of these new approaches.
These numerical models simulate transportation and sedimentation (or erosion) of the sediments in the basin on the basis of a description of the nature, from an estimation of the eustasy (sea surface variations recorded simultaneously on the whole land surface), of the subsidence (absolute displacement of the bottom of a sedimentary basin in relation to a fixed reference level in the course of time) and of the sediment supply at the boundaries of the basin studied.
Among these numerical models, the diffusive models have proved efficient through their use in many studies carried out notably for the petroleum industry in order to better (and more readily) identify zones likely to contain hydrocarbons. Such models are described in the document hereafter:
Rivenaes, J. C., 1988, Application of a Dual-Lithology, Depth Dependent Diffusion Equation in Stratigraphic Simulation. Basin Research, 4, 133–146, and in the following patents of the Assignee: U.S. Pat. No. 5,844,799 and French patent application 02/16,456 relating to methods of modelling sedimentary basins filling.
These deterministic numerical models are integrated in a procedure for calibrating their input parameters, referred to as “inversion procedure”, shown in FIG. 2. The inversion procedure is intended to adjust the parameters of the model so that the results provided thereby best fit the reality observed. The temporal evolution of the subsidence and of the sea level, that is of the accommodation, is among the parameters to be adjusted. The adjustment criterion for the model obtained is based, among other things, on the capacity of the model to reproduce the geometry, and notably the map of the sedimentary unit thicknesses.
This inversion procedure is in most cases of a “trial-and-error” type, as described for example in the aforementioned French patent application 02/16,456. It can also be automated as described, for example, in French patent 2,776,393 of the Assignee, which relates to a stratigraphic reservoir modelling method, or in the following publication for example:
T. A. Cross and M. A. Lessenger, “Construction and Application of a Stratigraphic Inverse Model”, in: Numerical Experiments in Stratigraphy; Recent Advances in Stratigraphic and Sedimentologic Computer Simulations, Special Publication—Society for Sedimentary Geology. 62; pp. 69–83. 1999.
Whatever the method selected, implementation of this inversion procedure is generally very significant because it involves repeated use of the deterministic model. However, although it allows improving the agreement between what is observed and the model obtained, the inversion procedure does not ensure a satisfactory agreement between the result of the model and what is observed.
Notably, the thickness maps for the various sedimentary sequences are generally well constrained in stratigraphic modelling when they result from interpretation of a seismic survey. Now, exact adjustment of the deposition sequence thickness maps of the model to the maps given by seismic interpretation is absolutely not guaranteed by the inversion procedure. In practice, exact adjustment is nearly never reached. In the case of the trial-and-error type method, the success of the procedure entirely depends on the user's know-how and intuition. This also applies to an automatic procedure: it can never succeed if the data initially proposed by the user are too far from the solution.
FIG. 2 illustrates the prior art process which proceeds from beginning point 50 to point 52 where a non-stationary conventional model 52 is provided which is a function of inputs 54 of data for direct calculation which are diffusion coefficients and sediment supply and inversion parameters 56 which are initial paleotopography and an accommodation map which may possibly be sediment supply and diffusion coefficients. Point 52 may provide an output 58 of a lithologic composition map and a final topographic profile. The processing proceeds to point 60 where calculation of the criterion for adjusting the model to the thickness data which is a function of input 62 of data for adjustment calculation which is a thickness map. The process proceeds to criterion test 64. If a criterion test is not performed, the process proceeds to end point 66. If the criterion test 64 is performed, the process proceeds to an inversion loop in which the inversion loop proceeds to point 68 where parameters adjustment is performed which is an input to point 56 where the inversion parameters 56, as described above, are provided. The inversion adjustment 68 proceeds as an input to point 52 as described above.